Interpretive Summary: The native vegetation and pollinators of National Parks in the United States are under pressure from invasive plant species that have colonized and become naturalized. Currently, well over 600 species of alien plants exist in settings which are supposed to be primarily managed for preservation of native biodiversity. We know little of the effects of such invasions on beneficial insects such as native bees which are responsible for most of the pollination of native plants. Potential effects include competition between invasives and natives for pollinators with the result that visitation rates to native flowers drops, fewer seeds are produced and recruitment declines over time while that of invasives increases. To explore this possibility we compared the native bees visiting the flowers of the invasive plants tamarisk, and yellow and white sweet clover (Tamarix spp., Melilotus albus, M. officinalis) with those visiting seven native plant species in mid-summer at three sites in Capitol Reef National Park, UT. We found that as many species of bees visited the flowers of the three invasive plants as visited those of the seven species of natives and that, on average, invasive species were visited by twice as many species of bees as were natives. Species abundant on the flowers of invasives tended to collect both pollen and nectar suggesting that bees are using these resources to rear offspring. We argue that invasives likely compete with natives for pollinators only at the beginning of invasions and that during the subsequent process of naturalization, they are likely to selectively increase the carrying capacity and population size of native bees, particularly those of generalists and specialists of closely related plant species.

Technical Abstract:
We compared the native bees visiting the flowers of three species of invasive plants (Tamarix spp., Melilotus albus, M. officinalis) with those visiting seven native plant species in mid-summer at three sites in Capitol Reef National Park, UT, USA. Overall, as many species of bees visited the flowers of invasive plants as visited those of natives. On average, invasive species were visited by twice as many species of bees as were natives. With a single exception, visitors of invasives were mostly generalist bees, rather than specialists. Colletes petalostemonis, the only legume specialist recorded, was an abundant forager on the flowers of both species of Melilotus demonstrating that at least some specialist bees will move to invasive plants that are closely related to their usual hosts. Species abundant on the flowers of invasives tended to collect both pollen and nectar suggesting that bees are using pollen of Tamarix and Melilotus to provision their offspring. We argue that invasives neither facilitate the reproduction of uncommon natives nor consistently compete with natives for pollinators. Rather, they are likely to selectively increase the carrying capacity and population size of native bees, specifically generalists and specialists of closely related plant species.